Fuel Depletion

Question 1

Define burnable poison and state its use in the reactor.

Answer 1

It is a material used as a means of controlling the initial excess reactivity.

As the reactor operates, these materials absorb neutrons and “burn up.” The resultant nuclei have low neutron absorption cross sections, and therefore, have no further effect on the neutron flux.

Burnable Poisons are used in the core to:

• Provide more uniform power density
• Allow higher fuel enrichment at initial core load
• Provide neutron flux shaping

Question 2

Describe and explain the distribution of burnable poisons in the core.

Answer 2

There are more burnable poison rods near the center of the core. They help flatten the radial neutron flux distribution by suppressing the neutron flux in the higher enriched assemblies.

Question 3

Describe how and why boron concentration chages over core life.

Answer 3

The greater the concentration of boric acid, the more positive the temperature coefficient becomes. A positive MTC is limited by Technical Specifications.

• T.S. 3.1.3 The MTC shall be maintained less positive than 0.5 10^-4 delta rho per degree farenheit at less than or equal to two percent rated thermal power.

The maximum concentration for a negative coefficient is about 1,100 ppm.

As the core lifetime progresses, chemical shim is withdrawn from the core and the temperature coefficient becomes more negative.

Question 4

Given a curve of K_eff versis core age, state the reasons for the maximum, minimum, and inflection points.

Answer 4

• The excess fuel in the core at the BOL is a positive reactivity. The first effect on k_excess is a drop in magnitude due to the buildup of the fission product poisons, samarium and xenon to their equilibrium levels.
• The next major effect on k_excess is the burnout of the burnable poisons. From the 25th day until about one-third cycle, the excess reactivity of the core will increase, but at an increasingly slower rate.
• After this, the reactivity loss of the fuel burnout becomes the predominant factor influencing k_excess. From this time on, only negative reactivity is being added to the core due to fuel depletion. At this time, the burnable poison has been depleted to a negligible factor. Fission product poison buildup (other than Xe and Sm) and the fuel depletion cause the excess reactivity to decrease at an increasing rate for the rest of the cycle. Therefore, k_excess will decrease at an increasing rate from the one-third cycle point to the end of the fuel cycle.

Question 5

Draw the curve for K_excess over core life.

Answer 5

Question 6

Define the fuel exposure term - MWd/MTU

Answer 6

Used to describe the amount of energy released per unit weight of the fuel and is expressed in megawatt days per ton (or metric ton) of uranium.

Question 7

What prevents the Moderator Temperature Coefficient from being too positive at BOL?

Answer 7

Use of borosilicate glass rods reduces the initial boron concentration required on refuel.